1. Field of the Invention
This invention relates to heat-hardenable liquid film-forming compositions having fine particulate matter dispersed therein. In particular, this invention relates to those of such compositions which are used for coating substrates with a magnetizable layer to prepare memory devices such as, for example, magnetic memory discs (both hard and flexible), magnetic tape, and the like.
2. Description of the Prior Art
A variety of magnetic coating compositions are known to be useful in the fabrication of magnetic memory elements. Examples are those disclosed in Alles et al., U.S. Pat. No. 3,009,847 (Nov. 21, 1961); Higashi et al., U.S. Pat. No. 3,474,073 (Oct. 21, 1969); Akashi et al., U.S. Pat. No. 3,689,317 (Sept. 5, 1982); Brilovich et al., U.S. Pat. No. 4,049,566 (Sept. 20, 1977); Lohoff, U.S. Pat. No. 4,122,234 (Oct. 24, 1978); and Ogawa, et al., U.S. Pat. No. 4,320,159 (Mar. 16, 1982).
These and similar compositions generally comprise fine particles of a magnetizable solid, such as iron oxide, dispersed in a solvent-polymer solution. To prepare the memory element, a thin film of the dispersion is applied to a substrate material, which may either be rigid, such as aluminum, or flexible, such as a polyester film (MYLAR film, for example). Typical methods of application include dip coating, spray coating, reverse roll coating, transfer roll coating, and spin coating. The coated substrate is then heated to evaporate the solvent and to cure the polymer, leaving a hardened film with the magnetizable particles embedded therein.
Some of these compositions have a relatively soft surface hardness upon curing. As a result, elements made with such compositions, when employed in tape recorder applications, are highly susceptible to frictional wear resulting from direct contact with the recording and reproducing heads. This shortens the useful life of such tapes.
A similar problem arises in elements used in rotating disc file memories. These discs are typically rotated at extremely high speeds, while separated only by a thin cushion of air from one or more aerodynamically supported read/write transducer heads. Despite the flotation effect, intermittent contact normally occurs between the surfaces of the transducer heads and the surface of the rotating disc. The abrasion resulting from this contact can sharply reduce the useful life of the disc.
In high-speed applications in general, including both linear tape drives and rotating disc memories, the acceleration forces experienced by the magnetizable particles embedded in the film layer when certain compositions are used are often high enough to cause ejection of the particles from the layer. In addition to weakening the magnetic susceptibility of the layer, this particle migration causes an accumulation of the particles on the surface of the associated transducer. The result is an impairment of the aerodynamic characteristics of the transducer and its associated support, and the "crashing" of the head into the magnetic recording surface, destroying the utility of the entire memory element.